Spring is unpredictable in Kansas, where I live. Some years, it makes an early appearance: we’re digging in storage tubs for summer clothes because the temperature’s approaching 80 degrees. The kids are begging me to set up the baby pool. Single digits aren’t unheard of, either, though. This year, we’ve had some days around 60, but more that are cold and wet or even snowy. In my un-insulated 1955 house, that translates into high and unpredictable energy bills. It’s a great time of year to daydream—and this year, finally, plan—ways to make this place more efficient.
For years, I’ve been fascinated with the idea of energy self-sufficiency; however, it seemed anything but attainable. A decade ago, we spent three years in a home with a solar water heater and a wood burning stove. We experimented with watering our garden with gray water from the laundry. That started me dreaming about a home that could meet all of its own energy needs, but at the time I didn’t know about all the other people who were starting to work to realize that vision of a net-zero house.
Defining Net-Zero House Terms
Put simply, a net-zero home is a house that is able to produce at least as much energy as it uses. ZERH stands for “Zero Energy Ready Home”—a home that’s built with energy conservation in mind, and ready for additional energy-producing improvements, such as solar panels.
Of course the utility bills are a great place to start, but there’s a universal, handy resource for rating how much energy a home requires: the HERS Index, or Home Energy Rating System. For a little perspective, a newly constructed home built to code will have a HERS rating around 100, while older homes may be closer to 140.
Before accounting for solar or other renewable energy sources, a new, well-constructed, energy efficient home should have a score of around 40-50. With correct use and maintenance, combined with solar, such a home will be able to achieve a HERS Index score of 0, becoming a net-zero house.
“Passive House” (or Passiv Haus) is another term you may come across. While not identical to a net-zero house, they certainly overlap, especially in building techniques. The following video is very helpful in understanding both passive house standards (which originated in Europe) and ZERH. While the HERS index has not been fully adapted to rate passive houses yet, projects like this one that have been assigned a HERS rating have come in below zero—net positive!
Sealing the Envelope: A Net-Zero House Starts Here
“Build tight and ventilate right” is a slogan of the green building movement for good reason. Because a home that isn’t airtight is wasting energy and money constantly, and increasing the carbon footprint of its occupants.
You’ll hear a lot about a home or building’s “envelope” in construction circles. The simplest definition of envelope is that it’s the outside of the house. It includes the foundation, walls, roof, windows, and doors—everything that comes between the occupants and the elements. And it’s where comfort and energy efficiency (or discomfort and inefficiency) begin.
All aspects of the envelope are affected by “thermal bridges.” These are points of contact between the inside and outside, where heat and cold are able to move between the inside and outside of the building. One example would be the glass in an older single-pane window; heat or cold easily passes through, affecting efficiency and comfort. Also, traditional wall studs can be thermal bridges because they are in contact with both the interior and exterior walls. Net zero and passive house construction, though, use building techniques that reduce or eliminate common thermal bridges, keeping temperatures stable and comfortable.
While it’s important to pay attention to every component of the envelope, the places where these different components intersect deserves at least as much attention in the net-zero home. Seams and corners are sealed up tightly to keep the interior comfortable and energy use down.
A New Take on Walls
Wall construction is next-level in a net-zero house, using the best method for each situation to build walls that keep the weather out.
One approach to constructing highly insulated walls is to actually build two walls, leaving space of as much as twelve inches for insulation in between. Another method is to use exterior rigid insulation over a single wall.
Green Building Advisor’s article on choosing the right wall assembly offers a lot of helpful technical information on the pros and cons of different wall types, as does their blog post on choosing a superinsulated wall system.
The Two Sides of a Roof
The primary job if your roof has to do is keep water out. Next, it should save energy by holding heat in (or out of) the house. So insulation in the attic, and even insulated roof panels, can be part of that equation.
No, Three Sides (In a Net-Zero House)
But with solar technology, your roof can also become a source of energy.
You will have to work within the constraints of your lot, but it will help if your roof has as much south-oriented area as possible. You can get very scientific about your home’s roof alignment, but it’s reasonably forgiving. For a roof that’s not south facing, the pitch can have a significant influence on solar efficiency. “If you had an optimum south-facing roof, you’d be at 98% production; if you had an east-facing roof at a 35-degree pitch, you’d be at 80% production. And if you were east at a 21-degree pitch, you’d have about 84% because there’s less shading by the roof. So that more flat roof helps you out a bit,” notes Charlie Morgan of Eastern CT Solar in this helpful article from Builder Online.
West-facing roofs also have solar potential for the net-zero house. While they give less power overall, they deliver the most during peak-usage evening hours, working hardest when people are coming home from work and using more energy (and when the sun is dishing out the most heat in summer, depending on what part of the country you’re in).
There should be as little shade as possible on the roof during prime solar hours. (Try planting trees on the north side instead, where they’ll provide a windbreak in winter, further enhancing your home’s efficiency. Keep reading for a full section on landscaping.)
Check out this article from The Solar Nerd for some great particulars to help you understand solar specifications.
Closing Doors and Opening Windows
According to Energy.gov, “Heat gain and heat loss through windows are responsible for 25%–30% of residential heating and cooling energy use.” (Also check out Energy.gov’s handy guide for window performance ratings.) Both the windows themselves and the installation are factors in how efficient they’ll be. Because heat and cold don’t just come through the glass, but can also come through air leaks around the outside of the window. So clearly (pun intended), windows are going to be super important in the quest for a net-zero house. While standard windows may cost less up front than energy-efficient windows, they will cost you more in utility bills over the long haul.
And don’t forget to consider windows as the major source of daytime lighting for your home. Putting the right size windows in the right places will reduce the number of hours you’re dependent on electric lighting. Plus, you’ll feel better!
Doors, even when they’re shut, can be a source of cold and heat just like windows. Paying attention to the insulating qualities of the doors you choose will pay off over time. And installation, once again, is crucial in helping your doors live up to their energy-efficient rating for a net-zero house.
Envelope Components to Consider for Increasing Efficiency and Getting Your House to Net Zero
- Insulated corners
- Insulated headers over windows and doors
- More space for insulation at intersections of interior and exterior walls
- Multiple layers of insulation; e.g., combining blown-in wall insulation with rigid exterior foam
- Continuous XPS barrier/thermal break
- Air gap between insulation and siding (this depends on your climate)
- Super-insulated attic
- Generous overhang above windows and doors to help keep weather out
- OSB roof sheathing with taped seams
- Insulation in crawl space/basement
What’s On the Inside
Once your net-zero home is put together with a tight, well-insulated envelope, it’s time to consider the systems that keep the inside comfortable, as well as the various appliances you need to get things done.
Ventilation: A Breath of Fresh Air
Did you know that most of the fresh air in an older house comes from leaks? Without those old drafty doors and leaky windows, you do have to give more consideration to how fresh air can get in for good indoor air quality (IAQ) in a net-zero house. Mechanical Ventilation Heat Recovery Systems (MVHR Systems) are one way to bring fresh air into the home without losing heat that’s already been generated.
(And of course, even your super-efficient, perfectly installed windows can be opened when the weather is nice!)
HVAC: Heating, Ventilation and Air Conditioning for a Net-Zero House
Heat pumps are an extremely efficient way to cool and heat your home. This is especially true in warmer climates, but improving heat pump technology combined with superior insulation enable newer heat pumps to keep northern homes warm, too.
Put simply, heat pumps work by moving heat from one area to another: inside to outside for cooling, outside to inside for heating. Since moving heat requires less energy than creating it, it makes for an efficient system. (Imagine running a standard air conditioner on a blazing August afternoon. Somewhere far from your home, fuel is being burned to create heat energy to generate electricity…to cool the house.)
Air source heat pumps pull heat out of the outdoor air to heat (and there is heat in the air, even when it feels cold outside) and release extra heat back into the outdoor air to cool the house. They are the most common type and easier to install in most houses.
Ground source heat pumps use heat from the earth to heat the house, and use the ground as a heat sink to release extra heat when cooling is required. While more efficient than air source heat pumps, they are more expensive to install and need a suitable location for burying the required underground pipes.
The familiar heat pump in the U.S. is a large outdoor unit that works with an indoor duct system, just like a traditional central air conditioner, except that it is also able to heat the house.
Ductless mini-split heat pump
But when Habitat for Humanity of Catawba, North Carolina set out to build an affordable net-zero house, they chose a mini-split heat pump for heating and air conditioning.
A mini-split heat pump works just like other heat pumps, but does away with traditional ductwork and uses small refrigerant lines instead, eliminating a common cause of energy loss. One outdoor unit is connected to indoor units in different rooms within the house, allowing individualized climate control for different areas. (It’s helpful to know that you don’t need an indoor unit in every room for this system to work efficiently.)
Sizing Your System
Whatever kind of heating and air conditioning system you install in your net-zero house, experts agree that bigger isn’t better. Make sure your system is the right size for the floor plan of your house–you’ll be more comfortable and save energy, too.
Where Does the Energy Go?
With your energy-efficient house all put together, it just wouldn’t make sense to bring in appliances that are going to guzzle up electricity or gas.
The U.S. Department of Energy gives an Energy Star rating to appliances that meet a high standard of efficiency. Here’s a link to their product finder for every home appliance you can think of (plus a few).
Water heaters get a special mention here because heating water comes right after heating and cooling living spaces as the biggest energy user. And there are some great energy efficient options, including systems that work together with your home heating system, solar units, and more. (Stacking functions=net-zero house win!)
When you plan for laundry, don’t forget to consider the possibility of adding a clothesline to your back yard. It will save electricity and keep extra heat out of your home in summer, plus you’ll get a little exercise (with a complimentary dose of vitamin D).
There’s also a handy gadget you can install on your dryer that allows you to keep the dryer’s heat inside your house during the winter, instead of blowing it directly outdoors. (You can even DIY one, if that’s your jam.)
Directenergy.com put together this handy chart to compare the cost of operating standard and energy star appliances.
|Average HouseholdBill Savings for 2015 ENERGY STAR vs. 2015 Non-certified Models||Cost Savings per Year||5-year Savings*||Average Lifespan||Lifespan Savings*|
|Clothes Washer||$40||$189||11 years||$415|
|Clothes Dryer||$16||$67||12 years||$160|
|Air Conditioners||$11||$55||9 years||$99|
|Air Purifier||$27||$119||9 years||$215|
Geothermal for Clean Energy
Geothermal energy has a lot of potential for eco-friendly heating and cooling of living spaces as well as heating water. That’s an article unto itself, but here’s an explanation from Popular Mechanics of how home geothermal systems work.
Love Your Net-Zero House Landscape
You might think of landscaping just in terms of curb appeal. But according to energy.gov, “On average, a well-designed landscape saves enough energy to pay for itself in 8 years.” Landscaping can be an important element in your quest for a net-zero house.
Heat for Free
Any concrete or asphalt around your home—driveways, sidewalks, etc.—is going to become thermal mass influencing the microclimate of your lot. As the sun shines on it during the day, it absorbs energy and heats up; all night, it releases that energy back into the air. Other sources of thermal mass might be large rocks in the landscape, paver bricks, etc.
You can use this to your advantage on the south side of a structure if the roof has enough overhang to shade it in the summertime and keep it cool. In winter, the angle of the sun will allow it to warm up and make the environment around the building warmer.
On the same principle, if summers are hot in your area, you’d probably want to avoid an un-shaded parking area or patio on the west side of the house.
Plants Are Cool
Foliage, on the other hand, helps keep the environment cool when the sun is beating down. Taller trees and shrubs can be a valuable source of shade, reducing the load on your air conditioner (and your wallet).
Trees that shed their leaves in winter are twice as nice—they offer shade all summer, and let the warm sunshine through their bare branches in the wintertime so it can help your heater out a bit.
Trees also affect the speed of wind across your property, making it warmer in the winter (and generally quieter and more pleasant).
Trees Are Awesome
If you’re reading this, you’re probably concerned about the environment at least a little bit. Beyond their direct benefits of aesthetics, comfort and savings on your energy bills, trees are a fantastic way to help the earth. Forty years from now, a tree you plant will have sequestered one ton of carbon dioxide from the air, among lots of other benefits!
A Net-Zero House: Counting the Cost
Will you pay more up front to build a net zero home or remodel to ZERH standards? Yes. But maybe not as much as you think you will. And the increased value of your home added to the savings in utilities and benefits to the environment make the decision easy.
This study by the Rocky Mountain Institute found that the additional up-front cost to build ZE or ZER homes was between one and eight percent higher than conventional building, depending on location.
Habitat for Humanity of Catawba, North Carolina spent an extra $6,000 to build a ZER home where the utility savings alone will make up for the difference within seven years. When you compare the increase in mortgage payments to monthly utility savings, though, the payoff is actually immediate. Here, a professional builder explains the cost of owning an efficient home:
“Here’s a real example of a 3,000 square-foot home with a $300/month average utility bill. If you spend $10,000 additional on the green aspects of the home, you can reduce that energy cost to $150 per month. At today’s mortgage rates, the $10,000 you spend costs you about $30 per month. You’ve saved $150 in utility costs and you’ve spent $30 to do it. Your positive cash flow that first month is $120, and it will be at least $120 a month after that. Whenever I’ve explained that to a customer, whether they’re buying a $100,000 home or $3 million home, they’ve never failed to embrace it and find great value in it.”
–T.W. Bailey Sr., president of Bailey Family Builders, Frisco, Texas
Don’t Forget to Decorate Your Net-Zero House Green
Green is trending in home interiors, too! While many standard options can be carbon-intensive and otherwise unfriendly to the environment, there are a growing number of companies out to change that. These innovators are offering floors, countertops, and more that are not only eco-friendly and cost competitive, but durable and beautiful, as well. Combined with low-VOC paints and finishes, these choices also impact your indoor air quality over the lifetime of your home.
The Future is Now
For a list of zero-energy ready homes in every U.S. climate classification, check out this page of virtual tours from the Department of Energy. Click on the name of any home to see photos and comments from builders and homeowners, plus specifications and links to even more details about each home’s construction.
We at Attainable Home are looking forward to a day when everyone in this country wakes up to a brighter future: in a home that is protected against rising energy prices, that is comfortable year-round, with a mortgage that’s affordable, and a clean, healthy environment that promotes well-being and productivity. A future where our need for energy isn’t wrecking our environment because we are able to supply what we need from clean, renewable sources.
Zero energy construction is the future of home building and renovation. With improving technology, decreasing costs and growing awareness on the part of consumers and contractors, that future is now.